1. Field of the Invention
The present invention relates to an improved air heater for use in association with industrial heat treating furnaces, kilns, cracking units in refining processes or similar installations which utilize radiant heating tubes.
2. Description of the Prior Art
The use of radiant heating tubes in kilns, cracking units, and heat treating furnaces is already well-known, the radiant heating tubes conveying heated air through the heat treating furnace in order to increase the temperature of the furnace enclosure. The principle of operation of a radiant heating tube used for heating an industrial furnace is that the outer surface of the tube provides heat by radiation into a furnace chamber. In order to achieve this efficiently, it is necessary to provide a source of radiant heat inside the tube, whereby the tube is heated to incandescence. When combustible fuels are utilized, this requires that there be a flame of a long, luminous nature, within the radiant heating tube since the luminosity is the property by which energy is radiated more or less uniformly to the internal tube surface. Any other method in which a flame is not produced within the radiant heating tube, such as in the case when there is only hot gas in the tube, relies more on convective heat transfer from the outer surface of the radiant heating tube to the furnace chamber. However, convective heat transfer becomes increasingly inefficient in this application as temperatures rise above 1000.degree. F. As a result, hot gas cannot be utilized in radiant heating tubes in applications where it is necessary to increase the temperature of a furnace enclosure above 1000.degree. F.
U.S. Pat. No. 3,930,382 to Hovis et al is one example of a system in which hot gas, as opposed to a flame, is directed through the radiant heating tubes of a furnace. In particular, in the embodiment of FIG. 7 of the patent, Hovis et al rely on the circulation of hot combustion gases alone via a hot gas line into hot gas feed headers coupled to radiant heating tubes. However, hot gases alone in the radiant heating tubes will not result in the tubes being heated to incandescence fast enough. As a result, little radiant energy is transferred from the outer surface of the radiant heating tubes to the interior of the furnace enclosure, thus limiting the temperature to which the furnace enclosure can be increased.
U.S. Pat. No. 2,041,930 to Houlis relates to a baking oven fired by fuel, diesel oil, or gas. As in the case of the Hovis et al patent referred to above, Houlis does not teach the creating of a flame within a radiant heating tube, but rather discloses use of hot gases only entering an enlarged pipe member 17', the latter being of greater diameter than the exit end of the fire box which is situated therein.
In known installations in which radiant heating tubes are heated to incandescence by means of a fire or flame therein, the fire or flame is created by utilizing propane or natural gas as the fuel source. However, the Applicant has found that there are two essential drawbacks with the known system in which propane or natural gas is utilized. Firstly, the use of propane or natural gas is an expensive source of energy in the known radiant heating tube systems as none of the gases dispersed from the exit end of the radiant heating tube, which are at approximately 1,200.degree. F., are utilized in any manner insofar as heat content thereof is concerned. As well, there is a potential shortage of natural gas in North America which could render the future use of natural gas as a source of fuel prohibitive.
One of the reasons that natural gas or propane is utilized in existing radiant heating tubes for heat treating furnaces and cracking units despite the above drawbacks is that the use of oil fired directly into a radiant heating tube would result in carbonizing of the radiant heating tube due to soot or dirt formed as the flame within the tube contacts the inner surface of the same. Hot points caused by carbon deposits in the radiant heating tube could result in burning out of the tube, necessitating replacement thereof, which could be quite expensive in the case of stainless steel tubes. As a result, oil has not been successfully used to date as a source of fuel in smaller diameter radiant heating tube installations in which a flame is produced in the radiant heating tube without also utilizing substantial quantities of excess air to avoid carbonizing. However, the excess air used must be heated to the temperature of the radiant heating tube, necessitating an additional consumption of fuel to heat the excess air and excess air results in short flame which does not radiate as well.
U.S. Pat. No. 2,796,118 to Parker et al discloses an arrangement in which a burner for tube firing is connected directly to a radiant heating tube in such a manner that a flame is produced within a radiant heating tube. However, in the case of the Parker et al patent, combustion takes place within the radiant heating tube utilizing a flame retention nozzle situated therein. In order to prevent carbonizing of the radiant heating tube, Parker et al disclose the use of a flow of secondary air adjacent the walls of the radiant heating tube to prevent contact thereof by the flame within the tube. Accordingly, substantial quantities of excess air must be employed in order to guide the flame within the radiant heating tube. However, as noted above, the use of excess air necessitates the heating of the air to the temperature of the radiant heating tube, thus requiring the use of additional oil to effect the heating of the excess air. While this additional consumption of oil might have been acceptable twenty years ago, such is certainly not the case today in the United States and Canada where efforts are being made to minimize the consumption of oil and reduce costs to as great an extent as possible. Additionally, the use of the excess air passing through the radiant heating tube at high velocity requires the use of a high energy fan, necessitating a further consumption of energy in the case of the Parker et al arrangement.
Further, with the Parker et al arrangement, it is impossible to ensure complete combustion of the oil in radiant heating tubes, thus resulting in carbonization of the radiant heating tubes and possible burning out of the tubes. In particular, unburned particles of oil from the flame retention nozzle would fall directly onto the walls of the radiant heating tube resulting in such carbonization. Since even a minimum amount of carbon deposits forming hot points could result in burning out of the radiant heating tubes in a short period of time, the use of the Parker et al arrangement could present serious problems in use insofar as replacement of expensive stainless steel radiant heating tubes is concerned.
British Specification No. 1,412,810, granted to Borg-Warner Corporation, relates to a heat exchanger in which a flame is directed into a tube, the flame being produced preferably by a gas fired burner. As in the case of the Parker et al patent, this patent necessitates the use of excess air in order to create a swirling action of the products of combustion in the U-shaped tubular heat exchange element, thus requiring additional fuel to heat the excess air to the temperature of the tube. Additionally, the Borg-Warner Corporation patent limits the flame created to the combustion chamber, with the products of combustion flowing through the heat exchange tube. Accordingly, it is apparent that the same is not directed to use of oil to create a flame within a radiant heating tube without carbonizing of the latter.